The possibility of identifying the geographical location of a target node, such as e.g. a user terminal in a wireless network, has enabled a large variety of commercial and non-commercial services, such as e.g. navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by the application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, i.e., Federal Communications Commission (FCC) Enhanced 911 (E911) in USA.
In many environments, the position of a node may be estimated by using positioning methods based on Global Positioning System (GPS). In some networks, Assisted-GPS positioning (A-GPS) may be employed to assist user terminal in order to improve the terminal receiver sensitivity and GPS start-up performance. GPS or A-GPS receivers, however, may not be available for all user terminals. Furthermore, GPS may sometimes fail when the target node is situated in indoor environments and/or urban canyons. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by 3rd Generation Partnership Project (3GPP).
With OTDOA, a terminal measures the timing differences for downlink reference signals received from multiple distinct locations. For each measured neighbour cell, the user terminal measures Reference Signal Time Difference (RSTD), which is the relative timing difference between the neighbour cell and a reference cell. The user terminal position estimate may then be obtained as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry may be used to solve for two coordinates of the user equipment and the receiver clock bias. In order to solve for position, precise knowledge of the transmitter locations and transmit timing offset is needed. Position calculation may be conducted, for example, by a positioning server, such as an Evolved Serving Mobile Location Centre (E-SMLC) in Long Time Evolution (LTE), or the user terminal. The former approach corresponds to the terminal-assisted positioning mode, whilst the latter corresponds to the terminal-based positioning mode.
To enable positioning in LTE and to facilitate positioning measurements of a proper quality for a sufficient number of distinct locations, new physical signals dedicated for positioning, such as e.g. Positioning Reference Signals (PRS), have been introduced and low-interference positioning subframes have been specified in 3GPP.
PRS are transmitted from one antenna port (R6) according to a pre-defined pattern. A frequency shift, which is a function of Physical Cell Identity (PCI) may be applied to the specified PRS patterns to generate orthogonal patterns and modelling the effective frequency reuse of six, which makes it possible to reduce neighbour cell interference on the measured PRS and thus improve positioning measurements. Even though PRS have been specifically designed for positioning measurements and PRS in general may be expected to have better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g., Cell-specific Reference Signals (CRS) could in principle also be used for positioning measurements.
Since for OTDOA positioning PRS signals from multiple distinct locations need to be measured, the user equipment receiver may have to deal with PRS that are much weaker than those received from the serving cell. Furthermore, without the approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the user terminal would need to do signal search within a large window which would impact the time and accuracy of the measurements as well as the user terminal complexity. To facilitate user terminal measurements, the network transmits assistance data to the user terminal, which includes, among the others, a neighbour cell list with PCIs, the number of consecutive downlink subframes, PRS transmission bandwidth, etc.
To facilitate position calculation and estimate the quality of positioning measurements, some quality metric for positioning measurements, which are RSTD in LTE, may be an advantage. The estimated quality may then be delivered to the network element which uses this information, such as the positioning node in the network in the case of user terminal-assisted positioning. The positioning node may in LTE comprise, for example, an E-SMLC and/or the user-plane positioning node SUPL Location Platform (SLP), or a base station or Radio Network Controller (RNC) in some other systems, etc. The estimated quality may however also be delivered to the terminal or derived and used by the terminal itself, e.g., in the case of terminal-based positioning.
Using standard deviation of measurements as a measurement quality metric is very common in the research literature. It has also been standardized for other systems, e.g. Universal Terrestrial Radio Access (UTRA), although the OTDOA-like positioning method in UTRA has not been used yet in real networks. Furthermore, the approach has some practical issues discussed later, which reveals a need of an alternative approach and motivates the importance of the disclosed solution.
As standardized for UTRA in [3GPP TS 25.331], for each measured cell in a plurality of cells comprising the reference and measured neighbour cells, the user terminal obtains the “user terminal positioning OTDOA quality” information which comprises e.g. the information shown in Table 1.
TABLE 1User Equipment positioning OTDOA quality in UTRAInformationElement/Type andGroup nameReferenceSemantics descriptionStdBitStd Resolution field includes theResolutionstring(2)resolution used in Std of OTDOAMeasurements field. Encoding ontwo bits as follows:‘00’10 meters‘01’20 meters‘10’30 meters‘11’ReservedNumber ofBitThe ‘Number of OTDOA measurements’OTDOAstring(3)field indicates how many OTDOAMeasurementsmeasurements have been used in the UE todetermine the sample standard deviation ofthe measurements. Following 3 bit encodingis used:‘001’ 5-9‘010’10-14‘011’15-24‘100’25-34‘101’35-44‘110’45-54‘111’55 or moreSpecial case:‘000’: In this case the field ‘Std of OTDOAmeasurements’ contains the std of thereported SFN-SFN std value = √E[(x − μ)2],where x is the reported value and μ = E[x] isthe expectation value (i.e. the true value) of x.This std may be used irrespective of thenumber of measurements and reporting of thenumber of measurements is not needed.Also other measurements such as Ec/No orRx levels may be utilised in this case toevaluate the ‘Std of OTDOAmeasurements’ reported in this IE.Std of OTDOABitStd of OTDOA Measurements field includesMeasurementsstring(5)sample standard deviation of OTDOAmeasurements (when number of measure-ments is reported in ‘Number of OTDOAmeasurements field’) or standard deviation ofthe reported SFN-SFN otd value =√E[(x − μ)2], where x is the reported value andμ = E[x] is the expectation value (i.e. the truevalue) of x (when ‘000’ is given in ‘Numberof OTDOA measurements’ field). Followinglinear 5 bit encoding is used:‘00000’0 - (R*1-1) meters‘00001’R*1 - (R*2-1) meters‘00010’R*2 - (R*3-1) meters. . .‘11111’ R*31 meters or morewhere R is the resolution defined by StdResolution field. E.g. R = 20 m correspondsto 0-19 m, 20-39 m, . . . , 620 + m.
OTDOA measurement quality in LTE and the signalling means for it are currently under active discussions in 3GPP. In 3GPP TS 36.355; E-UTRA; LTE Positioning Protocol (LPP), the details of the RSTD quality metric are not yet specified.
One of the proposals has been to adopt the UTRA specification for E-UTRAN with a minor change in the “std Resolution” parameter definition to allow resolutions of 5, 10, 20, and 30 meters. Rather than use absolute quality testing, another proposal is to adapt a method used for testing Channel Quality Indicator (CQI) reporting, i.e., in a given, e.g. fixed, condition determine the reported median value and determine that the sufficient amount of reports fall within a predetermined range. It has been observed that standard deviation is not very appropriate in practice as a quality metric because it is not be measured directly and therefore it has been proposed to remove the means for its signalling from LTE Rel. 9.
The previously existing solutions may be split into the following three groups:                1) solutions that drop the quality metric;        2) solutions using a relative quality metric reflecting the quality of reports but not the absolute quality of measurements; and        3) solutions using measurement standard deviation as a quality metric.        
Solutions that drop the quality metric have the following disadvantages:                1) In positioning, it is typically not only the position that needs to be estimated, but also the uncertainty of the position estimate needs to be provided, e.g., according to FCC requirements or when ensuring positioning quality of service, and the RSTD measurement quality is necessary for both, i.e., for the position estimate and its uncertainty.        2) The network needs some information to sort out unreliable measurements reported by the user terminal and select the combination of cells with the best geometry for inclusion in position calculation to achieve better position estimate for the user terminal.        
Solutions that use a relative quality metric have the following drawbacks:                1) User terminal positioning reports must follow predefined formats which would become impossible with CQI-type reporting quality testing since the absolute measure of RSTD error is necessary to estimate the position error in absolute units.        2) The pre-defined formats and the absolute measure of the position estimate are also required by FCC.        3) Only the formats with absolute uncertainty information may be given a Quality of Service (QoS).        4) The reported RSTD quality is to be used, for example, by the network for selecting the appropriate set of cells for inclusion in position calculation where one of the criteria is the measure of the absolute quality of RSTD which allows for selecting cells with a larger relative error when its absolute value is still acceptable.        
Solutions using measurement standard deviation as a quality metric have at least the following practical problems:                1) For any statistical characterization of a measurement, the size of the sample set must be sufficiently large, which may be hard to achieve for PRS due to e.g. large periodicity.        2) Since the standard deviation characterizes the deviation of a random variable around its expected value, the metric is not well suited when the reported measurement is not the algebraic average, but, for example, moving average.        
There is, therefore, an increasing need and desire for improvements in measuring timing signals for positioning, and for reducing errors and uncertainties in relation to such measurements.